The invention relates to a gas exhaust system for an internal combustion engine, in particular for vehicles.
Gas exhaust systems for internal combustion engines comprise an exhaust manifold in which the connector pipes from the individual combustion chambers are united to form a gas exhaust pipe. Generally, the gas exhaust pipe is directed rearward below the floor subassembly of the vehicle. To clean the flue gas, a catalyst with one or several catalyst elements is arranged in the gas exhaust pipe under the floor subassembly of the vehicle.
The catalytic effect of a coating on a catalyst element becomes active only above a relatively high operating temperature. When the distance between the exhaust manifold and the catalyst is relatively great, the operating temperature of the catalyst is reached only some time after the starting of the internal combustion engine (e.g. after two minutes). Since many vehicles are primarily used for short distance trips, the catalyst has not reached its required operating temperature over a great part of its operating time.
In order to reduce the warm-up time of the catalyst, i.e. the time until it reaches the required operating temperature, additional heating means are used, for example inductive heating of the catalyst, or heating the flue gas flow with gas or gasoline burners or heat accumulators located a short distance in front of the catalyst. The implementation of additional heating means results in an increase in weight, space requirement and cost.
In order to reduce the warm-up time of the catalyst, the catalyst may be located as close as possible to the exhaust manifold. Such an arrangement of the catalyst allows the required operating temperature to be reached very soon after the starting of the internal combustion engine. With this arrangement of the catalyst close to the exhaust manifold unburnt combustion gas mixtures may deposit in the catalyst and be ignited there. The resulting high combustion temperatures, as well as the sudden increase in pressure may cause damage to the catalyst. Pressure variations in the flue gas flow that occur near the exhaust manifold and caused by the delays between the outlet strokes of the combustion chamber, result in a degradation of the efficiency of the catalyst. Further, the distribution of the flow over the cross section of the exhaust pipe is strongly inhomogeneous just behind the exhaust manifold, seen in the flow direction. Due to the inhomogeneity of the flow, local overloads occur in the catalyst that may damage or destroy it.